It has long been known that the application of a stress can effect both the forward and reverse characteristics of P-N diodes in semiconductors. However, the prior art has required either very high hydrostatic pressures (on the order of 5000 to 10,000 PSI) or some form of external stress concentration (a sharp point which presses on the junction coupled with a large force collector).
In the present invention, the use of internal P-N junctions in porous semiconductors will allow large changes in both the forward and reverse characteristics at much lower applied stresses without the need for external stress concentrators i.e. outside the semiconductor structure. Porous silicon as formed by anodic reaction can consist of a series of very thin tubular pores arranged at right angles to the original surface of the wafer. The thickness of the resulting silicon rods can be of the order of 10000 .ANG. or less (down to 50 .ANG. or less).
Moreover because of the very thin cross section (below 100 .ANG.) it has been postulated that charge carriers can be confined in "Quantum Wells" and as such, has the effect of increasing the measured energy gap of the porous material. Other structures with small cross sections but greater in "diameter" than 100 .ANG. are also useful because of stress concentration effects.
For example, a P porous P-N structure results from taking a wafer of semiconductor material of one doping type, (i.e. n type); partially converting the wafer to porous material of the requisite wall thickness and pore size; forming a plurality of parallel internal P-N junctions on the surface of the crystallites by diffusing an impurity of the opposite type and growing on the porous face, an external or epitaxial layer of an opposite type to the starting material, such that the external layer contacts the diffused layer of the porous material. When a pressure or stress is applied to the epitaxial layer or cover a significant amount of internal stress magnification will result depending on the degree of porosity and the size and shape of the crystallites. By control of the anodic reaction process used to form the internal pores, the degree of stress enhancement can therefore, be controlled.
As is well known, the application of stress can change conductivity in a semiconductor by two separate mechanisms:
a) Change of the energy gap. PA1 b) In multivalley semiconductors, change of the relative population of the various valleys. In a semiconductor with anisotropic effective mass (in momentum space) this will change the relative number of "heavy" and "light" carriers with higher or lower mobilities. The greater the stress, the more charge carriers are transferred from one valley to another.
Both of these effects are known to change the characteristics of P-N junctions. The subject matter of the present invention is directed at the use of porous semiconductors to enhance the aforementioned effects. The first effect, namely the direct change of energy gap under high external stress, will clearly be enhanced by the internal stress concentrations, however, the second effect (namely changing the distribution of light and heavy charge carriers) can be more profound.
In a Zener diodes; for instance, it is presumed that reverse breakdown occurs from tunneling. It is known that the breakdown could be effected by large external stresses, which presumably creates more light carriers which could more easily tunnel through any barrier. In porous material these tunneling processes can occur at much lower external stresses. In addition, in the region of "Quantum Confinement" an increase in the number of light carriers can make the confinement less effective thus lowering the effective energy gap. The same basic arguments can also apply to the forward direction of such a diode.
Thus by biasing a reverse biased diode very near the breakdown point, application of an appropriate stress can cause the diode to switch from a low to a high conduct state or vice versa. Use of porous material with (or without) internal P-N junctions will cause this to occur at relativity lower external stresses and obviate the need for costly complex external stress concentrators.
The change from low to high conductive electrical resistivity of a semiconductor when a stress is applied along certain crystallographic directions is known as the piezoresistive effect. The use of the piezoresistive effect in semiconductors has resulted in the construction of electromechanical force transducers, pressure transducers, signal devices, and microphones. For an example of a pressure sensor employing the piezoresistive effect, see U.S. Pat. No. 4,204,185 entitled INTEGRAL TRANSDUCER ASSEMBLIES EMPLOYING THIN HOMOGENEOUS DIAPHRAGMS, issued to Anthony D. Kurtz and Richard A. Weber on May 20, 1989 and assigned to the assignee herein. For an example of a piezoresistive force transducer see U.S. Pat. No. 3,995,247 entitled TRANSDUCERS EMPLOYING GAP-BRIDGING SHIM MEMBERS issued to Anthony D. Kurtz on Nov. 30, 1976 and assigned to the assignee herein.
Of late, there has also been renewed interest in SiC as a semiconductor material. Its wide band-gap, high thermal conductivity, high breakdown electric field, and high melting point make SiC an excellent material for high temperature and high power applications. In U.S. patent application Ser. No. 07/957,519, the present applicants disclose methods of forming a new semiconductor material, porous SiC by electrochemical anodization of monocrystalline SiC. It is expected that the microporous structure of porous silicon carbide exhibits the same capacity for quantum confinement as that of porous silicon.
It is an object of the present invention to advantageously utilize porous semiconductor materials, such as silicon and silicon carbide, in the fabrication of improved stress sensitive device which are economical to produce and which do not require the use of expensive, specially designed external stress concentrating elements.